It's standard, though not very formal. Its an idiom meaning "not as much", but usually with a degree of ironic understatement (so the first quote doesn't just mean "some feedback isn't of as much use" so much as "some feedback is downright useless or unwelcome"). Think of it as the exact opposite of saying, "Others, so much more so", etc. I've nothing on the origin of the idiom to post an answer, though.
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Jon HannaFeb 11 '13 at 11:19

The so much part is an NPI and has a reversed meaning, like the very in He's not very smart. There's usually also a specifically ironic or sarcastic intonation contour on Not so much that emphasizes the opposition.
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John LawlerFeb 11 '13 at 14:53

Think of other qualifiers moved to the end of sentences. "Second one, maybe." "Second one, almost certainly." "Second one, to a degree" "Second one, if it's yellow". "Not so much" has taken on a life of it's own, but it's not unique.
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kbelderFeb 11 '13 at 16:49

1 Answer
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The phrase is meant to be dismissive and is a somewhat current catch-phrase that has been used comedically by some television celebraties in the US. While looking for the source of this expression (one of which cited the origin as the "Brady Bunch", a highly popular and iconic sit-com from the 1970's), I came across this excellent article by Daniel Weiss from the Columbia News Service - the gist of which is that unlike other catch-phrases that have a brief moment in the spotlight such as "Been there, done that" and "yada-yada-yada", "not so much" has a certain style of being dismissive without being downright rude. It was compared to fashion in that one can put it on (i.e., use the phrase) to come across as humorous/dismissive.

Hi Kristina! I agree this article is helpful, but it would probably be better to link to it and then quote the most relevant portions, giving your summary or interpretation, rather than pasting the entire original article. See discussions on Meta about this
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aedia λFeb 11 '13 at 16:37